Preparation And Characterization Of High Crystallinity Rule Morphology Of Ni-mn Layered Double Hydroxides

Layered double hydroxides (LDHs), also well-known as hydrotalcite-like compounds or anionic clays, are typical anionic layered compounds. The LDHs have controllability of structure and diversity of composition, which provide extensive space for its development and application. LDHs can be applied as catalysts, adsorbents, functional additives, biological medicine materials ect. In recent years, the research on the synthesis of LDHs with high crystallinity and uniform morphology becomes very important. Because of the variable oxidation states, manganese is an interesting element not only as a component of power materials but also as an effective component in the metal oxide based catalysts for oxidation and combustion reactions. Up to now, it is seldom-reported to prepare LDHs materials containing transition metal manganese. Therefore, it is very significant to study the preparation, characterization and property of Ni2+-Mn3+ LDHs materials.In this work, Ni2+-Mn3+ LDHs materials with high crystallinity and uniform morphology have been prepared using urea as hydrolysis reagent and ammonium peroxodisulfate as oxidant. The effects of the reaction temperatures, Ni/Mn molar ratio, oxidant species and its amount on the crystallinity and shape of the obtained LDHs materials have been systematically studied.The main works consist of three parts:(1) Study on the preparation of Ni2+-Mn3+ LDHs at relative low hydrothermal treatment temperatureNi2+-Mn3+ LDHs is hydrothermally prepared at relative low treatment temperature (150℃) by using urea as hydrolysis reagent and (NH4)2S2O8 as oxidant. The obtained product shows the characteristic diffraction pattern of LDHs material and no peaks of impurity are discerned, but its crystallinity is low. The effects of oxidant species, the adding amount of oxidant and Ni/Mn molar ratio on the crystallinity and morphology of the obtained LDHs materials have been systematically studied. The experimental results show that LDHs with regular flower morphology can be obtained by using (NH4)2S2O8 as oxidant at Ni/Mn molar ratio of 4.(2) Study on the preparation of Ni2+-Mn3+ LDHs with high crystallinity and regular flower sphere morphologyThe effects of Ni/Mn molar ratio and oxidant species on the preparation of LDHs materials with regular flower sphere morphology have been systematically studied. The experimental results show that Ni2+-Mn3+ LDHs with high crystallinity and regular flower sphere morphology has been hydrothermally prepared at 180℃for two days at Ni/Mn molar ratio of 4. When Ni/Mn molar ratio is smaller than 4, MnCO3 as impurity phase is detected. Without adding oxidant (NH4)2S2O8 in the reaction system, the obtained product is a mixture of MnCO3 and Ni(OH)2. (3) Study on the property of Ni2+-Mn3+ LDHs with high crystallinity and regular flower sphere morphology and its calcined productThe thermal decomposition behavior of Ni2+-Mn3+ LDHs between 200℃and 600℃has been investigated. The layered structures can be remained to 300℃, and the layered structure will collapse and form double metal oxide when the caicination temperature is in the range of 300℃and 400℃. A nickel-manganese spinel phase can be obtained when the calcination temperature reaches to 600℃. The BET surface area and the average pore size of the calcined products connect with the calcined temperature. The BET surface area of precursor Ni2+-Mn3+ LDHs is small; whlie the obtained material calcined at 400℃has the largest BET surface area.The materials obtained at different stages are characterized by XRD, SEM, XPS, FT-IR and atomic absorption analyses.